Category: Chimps and other apes

As animals get bigger, so do their brains. But the human brain is seven times bigger than that of other similarly sized animals. Our close relative, the chimpanzee, has a brain that’s just twice as big as expected for its size. And the gorilla, which can grow to be three times bigger than us, has a smaller brain than we do.

Many scientists ask why our brains have become so big. But Karina Fonseca-Azevedo and Suzana Herculano-Houzel from the Federal University of Rio de Janeiro have turned that question on its head—they want to know why other apes haven’t evolved bigger brains. (Yes, humans are apes; for this piece, I am using “apes” to mean “apes other than us”).

Their argument is simple: brains demand exceptional amounts of energy. Each gram of brain uses up more energy than each gram of body. And bigger brains, which have more neurons, consume more fuel. On their typical diets of raw foods, great apes can’t afford to fuel more neurons than they already have. To do so, they would need to spend an implausible amount of time on foraging and feeding. An ape can’t evolve a brain as big as a human’s, while still eating like an ape. Their energy budget simply wouldn’t balance.

These recent events all illustrate a broad human trait: we seek to punish people who do wrong and violate our social rules, even when their actions don’t harm us directly. We call for retribution, even if we have nothing specific to gain from it and even if it costs us time, effort, status or money to do so. This “third-party punishment” is thought to cement human societies together, and prevents cheats and free-riders from running riot. If you wrong someone, and they’re the only ones who want to sanction you, the price of vice is low. If an entire society condemns you, the cost skyrockets.

Do other animals do the same thing? It’s not clear, but one group of scientists believes that our closest relative – the chimpanzee – does not. Katrin Riedl from the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany found that chimpanzees will punish individuals who steal food from them, but not those who steal food from others. Even if the victim was a close relative, the third party never sought to punish the thief. These were the first direct tests of third-party punishment in a non-human animal, and the chimps got an F.

We normally think of nests as the creations of birds, but our ape cousins build nests too. Orangutans, gorillas, chimpanzees and bonobos all build tree beds, by weaving branches, twigs and leaves together into a bowl-shaped cradle. These nests may provide safety from predators, or help the apes to sleep warm.* But it seems that their main function is to provide a good night’s rest. Sleeping against a tree bough is hard on a large ape, and nests offer a more comfortable option.

Of all the apes, orangutans reputedly create the sturdiest and most elaborate nests. By studying the physical properties of these treetop bunks, Adam van Casteren from the University of Manchester has found that the apes are skilled engineers. As befits animals of their intelligence, they don’t just mash branches together. Instead, they seem to have an impressive amount of technical knowledge about their construction materials.

Orangutans build their nests between 11 and 20 metres up. Once they choose a good spot on a sturdy branch, they bend or break other branches in towards them, and weave them in place to create a basic foundation. On top of that, they add smaller branches to create a ‘mattress’. That’s the basic model, and some orangutans add deluxe features. They can create blankets, by covering themselves with large leafy branches, or pillows, by clumping such branches together. They can loosely braid branches above their heads to make a roof, or even create a secondary ‘bunk-nest’ over the main one.

Like all apes, orangutans construct new ones every day. This means that intrepid scientists have plenty of old discarded nests to study. Van Casteren, along with Julia Myatt from the Royal Veterinary College, found 14 such nests in the Sumatran rainforest. They hoisted themselves into the canopy, attached ropes to different parts of the nests, and lowered these to the ground where team members were waiting with force gauges. “Climbing up into the high canopy is breathtaking,” says van Casteren. “You enter an area of the forest that isn’t used to having humans hang around in it.”

Van Casteren found that orangutans use thicker branches in the structural foundation of the nest, and thinner branches in the mattress. The structural ones are four times stronger and four times more rigid, and they make the nest sturdy. The mattress branches are thinner and more flexible for comfort.

The orangutans also break the two types of branches in different ways. If you bend a dense branch, it will only break halfway – this is known as a “greenstick fracture (see below). That’s what van Casteren found in the structural part of the nest. Once broken like this, it’s surprisingly hard to fully snap a branch in two, even for a powerful animal like an orangutan. The trick is to twist the branch. The fracture extends outwards until the two halves come apart, producing two pieces with long tapered ‘tails’. Van Casteren filmed the apes using this technique, and the found plenty of the distinctive tailed branches in their mattresses.

There are plenty of questions about the nests left to answer. For example, orangutans don’t choose their trees randomly, and actually avoid the most common species. What’s special about the ones they pick, and does that factor into the properties of the nests? The apes also learn their craft from adults, so do immature orangutans build nests with less distinctive foundations and mattresses? Van Casteren also wants to look at the nests of other great apes, and of other architects such as beaver or birds, to see if he gets similar results.

But for now, his data already show that orangutans make sophisticated technical choices when they build their nests. He thinks that they account for the different properties of the materials in their environment, and use those properties to make bunks that are both safe and comfortable. While many studies of animal intelligence focus on the use of tools, he argues that nest-building is no less mentally demanding.

Roland Ennos, who was involved in the study, says, “I hope helps to show how the evolution of intelligence can be driven by the need to deal with the mechanical environment, rather than the prevailing orthodoxy that it’s only the social environment that’s important.”

* In writing this story, I stumbled across a wonderful study by Fiona Stewart from the University of Cambridge, who tested the value of chimpanzee nests, by sleeping in them. She spent several nights in Senegal either sleeping in newly made chimp beds or on the bare ground. She was warmer in the nests, and received fewer insect bites. She didn’t get any more sleep, but what she got was less disturbed. “Terrestrial animals, including hyenas, were more concerning during ground sleep, although snakes were always a concern,” she writes, in a wonderfully deadpan way. Van Casteren, however, never tried to sleep in the orangutan nests that he studied. They are higher than a chimp’s and he was “too worried about falling out mid-dream”.

We are like dwarves standing on the shoulders of giants. This metaphor, famously used by Isaac Newton, describes how humans build on what has come before. Everything in our culture is the result of knowledge and skills that have slowly accumulated over time. Without this “cumulative culture”, we wouldn’t have our deep scientific knowledge, rich artistic traditions, or sophisticated technology. Simply put, you can’t make a car from scratch – first, you need to invent the wheel.

Are we alone in this respect? Certainly, many other animals can learn knowledge and skills from each other, and many of them have cultural traditions. But Newton’s metaphor involves not just the spread of knowledge, but its gradual improvement. We build on the past, rather than just passing it along. As generations tick by, our culture becomes more complex. Do other species show the same ‘cultural ratchet’?

Lewis Dean from the University of St Andrews tried to answer that question by presenting human children, chimpanzees and capuchin monkeys with the same task: a puzzle box with three, increasingly difficult stages, each one building on the last.

The two apes above might look very similar to the untrained eye, but they belong to two very different species. The one on the right is a bonobo; the one on the left is a chimpanzee. They are very closely related but the bonobo is slimmer, with a smaller skull, shorter canines and tufts of lighter fur. There are psychological differences too. Bonobos spend more time having sex, and playing with one another. They’re less sensitive to stress. They’re more sensitive to social cues. And they are far less aggressive than chimps.

Many years back, a young researcher called Brian Hare was listening to the Harvard anthropologist Richard Wrangham expound on this bizarre constellation of traits. “He was talking about how bonobos are an evolutionary puzzle,” recalls Hare. “They have all these weird traits relative to chimps and we have no idea how to explain them.”

But Hare had an idea. “I said, ‘Oh that’s like the silver foxes!’ Richard turned around and said, ‘What silver foxes?’”

In the Olympic Games of Ancient Greece, long-jumpers would leap while carrying weights called halteres in their hands. From either a standing start or a short run, they swung the weights and leapt as their arms came forward. The halteres each weighed up to nine kilograms, and would have added around 17 centimetres to a 3 metre jump. Olympians first used the hand weights in 708 BC, but other apes were jumping with a very similar technique millions of years earlier – gibbons.

Gibbons are undisputed masters of the treetops, best known for swinging around at unfeasible speeds from their long, powerful arms. Their wrists contain ball-and-socket joints, which allow their entire body to easily pivot about their hands. This style of movement, known as brachiation, is a gibbon speciality (see video below). But these apes are also accomplished jumpers. Field scientists have watched them clear gaps as large as 10 metres.

Compared to most other animals, humans are unusual in our tendency to help each other out. We donate to charity. We give blood. We show kindness to strangers, even when we stand to gain nothing in return. This behaviour is so odd that the natural question arises: are we alone in such selflessness? And if any animal could help to answer that question, it’s the chimpanzee, one of our closest relatives.

Dozens of scientists study the behaviour of chimps, looking at how these apes act towards their peers. But the results of these studies have been frustrating for many in the field. People who watch captive and wild chimps have documented hundreds of cases of seemingly altruistic behaviour. They have seen individuals helping each other to climb walls, consoling each other after fights, sharing food, risking death to save companions from drowning, and even adopting the babies of dead and unrelated peers. Anecdotes like these suggest that chimps, like humans, behave selflessly towards each other.

But experiments have often shown otherwise. In some studies, chimps choose to help their peers retrieve out-of-reach objects rather than doing nothing. But when chimps have a choice between two equal actions – say, cashing in a token that leads to personal gain versus another that also benefits a partner – they only looked out for themselves. One paper bore the title “Chimpanzees are indifferent to the welfare of unrelated group members”. Another concluded that “chimpanzees made their choices based solely on personal gain”.

Collectively, these studies championed a view of chimps as reluctant altruists, who only act selflessly in response to pressure, and who generally don’t help unfamiliar chimps, “even when they are able to do so at virtually no cost to themselves”. But Frans de Waal from the Living Links Centre at Emory University thinks that this portrait is wrong. He says, “The authors of these studies moved from not finding evidence for prosocial choice to thinking they had proven its absence.”

De Waal thinks that the previous tests handicapped the chimps by putting them in situations that masked their altruistic tendencies. They couldn’t communicate, they had to cope with complicated equipment involving levers, and they often sat so far apart that they had little understanding of how their choices affected their fellows. With his colleague Victoria Horner, de Waal designed a new experiment to account for these problems. And, lo and behold, chimps spontaneously helped their partners, even without any prompting.

We are a cooperative ape, and a fair one. We work together to put food on the table and once it’s there, social rules compel us to share it around equitably. These two actions are tied to one another. In a new study, Katharina Hamann from the Max Planck Institute for Evolutionary Anthropology has shown that three-year-old children are more likely to fairly divide their spoils with other kids if they’ve worked together to get them.

The same can’t be said of chimpanzees, one of our closest relatives. Sharing comes less naturally to them, and it doesn’t become any more likely if they’ve worked together to get a meal.

Few groups of animals hold such special significance for us as the primates – the apes, monkeys, lemurs and more. This is the group that we are a part of. Its members are familiar and charismatic, but our evolutionary history is tangled and occasionally controversial.

Now, Polina Perelman has provided the most comprehensive view of the primate family tree to date. Her team sequenced genes from over 186 species, representing 90% of all the genera that we know of. Her tree confirms some past ideas about primate evolution and clarifies other controversies. It’s a story of island conquests, shrinking bodies, tangled branches and ancient relics. This slide show tells that story.

You’ve been asked to compete against some of your friends in a game of skill, but you realise something is amiss. They’ve been given precise instructions and details about the game’s mechanics. You’ve been given a couple of pieces and left to figure things out on your own. On this uneven playing field, no one could fairly compare your performance with that of your friends. This seems obvious, but it’s a problem that plagues a lot of research into the behaviour of humans and other animals.

Scientists will often test monkeys and apes with tweaked versions of psychological games that were originally designed to test humans. The goal is simple: understand the similarities and differences between our mental abilities and those of our closest relatives.

But these comparisons are tricky. Frans de Waal, who studies the behaviour of apes and monkeys, says, “Humans are tested by their own species and the apes by a different species (us). Humans understand everything the experimenter says or explains, whereas the ape needs to figure these things out based on experience. The paradigm really doesn’t permit the comparisons that have been made, especially the negative assessment of ape capacities.”

Sarah Brosnan form Georgia State University agrees. “As humans, we surely design tasks that are more intuitive to us than to other species. We don’t know whether humans perform differently from other species absent these advantages. Are human-specific abilities, including language, added ‘on top’ of other primate abilities, making us fundamentally similar in our outcomes. Or are we fundamentally different from the rest of the primates?”